Sediment provenance in the Palaeoproterozoic Rio Itapicuru greenstone belt, Brazil, indicates deposition on arc settings with a hidden 2.17–2.25Ga substrate

Abstract

Abstract A study of sediment provenance and tectonic setting was carried out for the first time on metasedimentary rocks of the Rio Itapicuru greenstone belt, Serrinha Block, Sao Francisco craton, Brazil, using a combination of LA-ICP-MS zircon U–Pb geochronology, whole-rock geochemistry and Sm–Nd isotope data. The protoliths of the studied phyllites, micaschists and biotite gneisses were classified mostly as mudstones and impure sandstones, i.e. shales and greywackes. The chemical index of alteration (CIA) ranges from 39 to 70 indicating moderate chemical weathering in source areas. Intermediate and felsic rocks (granite, granodiorite, rhyolite and andesite) are the likely main source rocks, and less often mafic rocks (basalt and gabbro). The depleted-mantle Nd model ages (TDM), most in the range 2.0–2.2 Ga, and the positive ɛNd values for the proposed deposition time (ca. 2115 Ma) suggest source rocks mainly in the greenstone belt. The U–Pb data on detrital zircon grains of four samples yielded 207Pb/206Pb age mostly in the timespan 2125–2237 Ma, with age populations clustering at 2164 ± 4 Ma and 2209 ± 4 Ma, and minor outliers at ca. 2112 Ma, 2270 Ma, and 2415 Ma. The results indicated that the metasedimentary rocks of the Rio Itapicuru greenstone belt have source areas mainly in Palaeoproteroic terranes such as the Rio Itapicuru and Rio Capim greenstone belts. The zircon populations between 2.17 Ga and 2.24 Ga are not found in the Serrinha block, thus requiring sources that are not known in the region. Diagrams of tectonic setting suggest that the sediments may have been deposited on continental island arc. Our data support a model in which basalts and arc granites of the Rio Itapicuru greenstone belt were accreted onto an unknown continental or arc margin and the entire pile collided with, and were thrust onto a microcontinent represented by the adjacent Archaean basement complex.